I'm still partway through S5, though I have been spoiled for some things, and honestly, it never crossed my mind that most of the princesses are actually born royalty! I think I kind of assumed that the gem picks the most appropriate (whatever that means to a gem) child each generation - the current ruler would obviously *like* it to be their biological child and probably have a head start genetically and/or by having their kids around the gems a lot (Glimmer and Scorpia) but do we see any evidence that any of the other princesses actually inherited their kingdoms from their parents?